Sammy/Yukinari

Japanese VA:
American VA:  Tara Jayne

Yukinari was brought to the present from forty years in the past by Serebii.  Yukinari was drawing one day when it noticed Serebii running from a hunter, so he decides to help it.  As the young Pokemon Watcher works to protect the time-travelling pokemon, he befriends Satoshi and forms a close bond with the young trainer. 
 

Iron Masked Marauder/Mamoru Vicious and Gareousu 

Japanese VA:  Shirou Sano

Vicious is an elite member of the Rocket-Dan in search of Serebii, the super-rare and super powerful pokemon.  He has supposedly developed a "Dark Ball," that turns the pokemon he's captured with it into evil creatures.  In fact, he even manages to turn the kind and gentle Serebii into a rampaging monster!  Using his mecha named Gareosu and his Dark Balls, Vicious is a trainer who will use any means necessary to achieve his goals.

???/Towa

Japanese VA:

Towa is the kindly woman from forty years ago who tells Yukinari about the "voice of the forest."  Forty years later, she is able to recognize Yukinari as the young boy who had vanished with Serebii all those years before. 

???/Miku

Japanese VA: 

Miku is Towa's granddaughter and 

Celebi/Serebii

Serebii, the "Myuu of Pokemon Gold/Silver/Crystal," is the star of the movie.  It is being hunted by Vicious because it is so rare and powerful.  Serebii has the ability to travel through time, and it uses this ability to bring Yukinari forty years into the future.

Suicune/Suikun

Suikun is the mysterious pokemon Satoshi met in the woods outside Wakaba Town.  It is the guardian of the forest that Serebii's been hiding out in, and it gets angry when Gareosu, Vicious' mecha, starts stomping around in its forest.  Suikun seems to be very similar to the Forest Guardian in the movie Mononoke-Hime.

Kecleon, Wailmer, and Azurill/Kakuleon, Whaleko, and Ruriri

These are the three new pokemon that make their video game debut in the Pokemon Ruby/Sapphire game.  Kakuleon is a chameleon that comes in different varieties, much like the Unown.  Whaleko is a whale pokemon that is often used to ferry trainers from one location to the next.  It's probably safe to say that it's not unlike a Kabigon in size, attacks, hit points, etc.  Ruriri is the pre-evolution of Mairiru, adding to the list of "baby pokemon."